date-time-utils: fall back to UTC when timezone is unrecognized

For whatever reason, libical doesn't support the Etc/UTC timezone.
Currently we crash gnome-calendar whenever libical doesn't support the
current timezone (as returned by GTimeZone). Let's fall back to UTC
instead.

Fixes #485

src/utils/gcal-date-time-utils.c

@@ -249,7 +249,9 @@ gcal_timezone_to_icaltimezone (GTimeZone *tz)
   tzid = g_time_zone_get_identifier (tz);
   ical_tz = i_cal_timezone_get_builtin_timezone (tzid);
 
-  g_assert (ical_tz != NULL);
+  if (!ical_tz)
+    ical_tz = i_cal_timezone_get_utc_timezone ();
+
   return ical_tz;
 }